A Novel Electroscrubbing Process for Gas Cleaning

Abstract

This doctoral thesis presents three years of research and development within a new electroscrub-bing process for biogas desulfurization. The process is designed to remove hydrogen sulfide (H2S) from biogas by using an active compound that is generated through electrochemistry. Independent on the downstream application of biogas, H2S has to be removed and all biogas plants have desulfurization processes implemented. However, the H2S removal technologies of today su˙er from either high operating expenses (OPEX) or residual oxygen (O2) in the puri-fied biogas. Electroscrubbers have several advantages, including no residual O2 in the purified gas, high eÿciency and no consumption of chemicals. The main disadvantage is that currently, there are no commercial electroscrubbing systems available. This research has been carried out within the MeGa-StoRE project. Part of the project objectives has therefore been to construct a pilot plant for field testing of the electroscrub-bing technology. The MeGa-StoRE project investigated second-generation upgrading of biogas through catalytic conversion of carbon dioxide (CO2) to methane (CH4). This process takes place over a catalyst, which is very sensitive to H2S. Electroscrubbing biogas for second-generation upgrading is a multidisciplinary scientific field. Research within this field require fundamental knowledge of electrochemistry, surface technology, gas scrubbing, process chemistry, biogas and catalytic processes. In this thesis, an empirical approach has been widely used for the research and development of the new electro-scrubbing process. Two laboratory and two campaign electroscrubbers have been designed, constructed and op-erated in this thesis. The laboratory electroscrubbers were used to investigate the fundamentals of the process and the dimensions required for the pilot plants. The campaign electrocsrubbers were used to investigate the process on real biogas and optimize the process parameters. The main conclusion of this thesis is that the new electroscrubbing process has proved to successfully remove H2S from biogas down to non-detectable concentrations. The process does not produce any waste as the two side products, H2 and sulfur, can be used within the biogas plant. The H2 can be used for the second-generation upgrading process, and the sulfur as a fertilizer. Scaling up the process to a pilot plant size proved possible. The campaign electroscrubbers did have a lower current eÿciency than the laboratory electroscrubbers, although this was partly compensated with an optimization of the electrical properties. Overall, the maximum power eÿciency of the laboratory electroscrubbers was found to be 16 ± 1.6% while the campaign electroscrubbers achieved 13.4 ± 0.9%. A power eÿciency of approximately 30% would make this electroscrubbing process the cheapest desulfurization technology based on OPEX for biogas plants. Both the laboratory and the campaign electroscrubbers proved capable of removing H2S down to non-detectable concentrations.